Separator for lithium secondary battery, and lithium secondary battery comprising same

ABSTRACT

The present invention relates to a separator for a lithium secondary battery, and a lithium secondary battery including the same. The separator includes a porous substrate, and a coating layer on at least one surface of the porous substrate, wherein the coating layer includes a binder including a (meth)acrylic copolymer including a first structural unit derived from (meth)acrylamide, and a second structural unit including at least one of a structural unit derived from (meth)acrylic acid or (meth)acrylate, and a structural unit derived from (meth)acrylamidosulfonic acid or a salt thereof; first inorganic particles; and second inorganic particles, wherein the first inorganic particles have an average particle diameter of 400 to 600 nm, the average particle diameter of the second inorganic particles is smaller than the average particle diameter of the first inorganic particles.

TECHNICAL FIELD

A separator for a lithium secondary battery and a lithium secondary battery including the same are disclosed.

BACKGROUND ART

A separator for an electrochemical battery is an intermediate film that separates a positive electrode and a negative electrode in a battery, and maintains ion conductivity continuously to enable charge and discharge of a battery. When a battery is exposed to a high temperature environment due to abnormal behavior, a separator may be mechanically shrinks or is damaged due to melting characteristics at a low temperature. Herein, the positive and negative electrodes contact each other and may cause an explosion of the battery. In order to overcome this problem, technology of suppressing shrinkage of a separator and ensuring stability of a battery is required.

In this regard, a method of increasing heat resistance of the separator by mixing inorganic particles having large heat resistance with an organic binder having adherence and coating them on the separator has been known. However, this conventional method may not sufficiently securing desired adhesive strength and in addition, is difficult to universally apply to separators having various sizes and shapes.

DISCLOSURE Technical Problem

A separator for a lithium secondary battery having high heat resistance and strong adhesive strength, and a lithium secondary battery including the same are provided.

Technical Solution

In an embodiment, a separator for a lithium secondary battery includes a porous substrate, and a coating layer on at least one surface of the porous substrate, wherein the coating layer includes a binder including a (meth)acrylic copolymer including a first structural unit derived from (meth)acrylamide, and a second structural unit including at least one of a structural unit derived from (meth)acrylic acid or (meth)acrylate, and a structural unit derived from (meth)acrylamidosulfonic acid or a salt thereof; first inorganic particles; and second inorganic particles, wherein the first inorganic particles have an average particle diameter of 400 to 600 nm, and the average particle diameter of the second inorganic particles is smaller than the average particle diameter of the first inorganic particles.

Another embodiment provides a lithium secondary battery including a positive electrode, a negative electrode, and a separator for a lithium secondary battery between the positive electrode and the negative electrode.

Advantageous Effects

A lithium secondary battery including a separator for a lithium secondary battery having excellent heat resistance and adhesive strength may be implemented.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a lithium secondary battery according to an embodiment.

FIG. 2 is a SEM measurement result of inorganic materials in a separator according to an embodiment.

FIG. 3 is a graph showing a volume distribution weighted distribution particle size distribution curve of a composition for forming a coating layer of a separator according to an embodiment.

FIG. 4 is a photograph showing a change result after wet shrinkage of the coated separator according to Comparative Examples 4 to 8.

FIG. 5 is a graph showing wet heat shrinkage in a transverse direction (TD) according to the content of the second inorganic particles.

DESCRIPTION OF SYMBOLS

-   -   100: lithium secondary battery     -   112: negative electrode     -   113: separator     -   114: positive electrode     -   120: battery case     -   140: sealing member

MODE FOR INVENTION

Hereinafter, embodiments of the present invention are described in detail. However, these embodiments are exemplary, the present invention is not limited thereto and the present invention is defined by the scope of claims.

In the present specification, when a definition is not otherwise provided “substituted” refers to replacement of hydrogen of a compound by a substituent selected from a halogen atom (F, Br, Cl, or I), a hydroxy group, an alkoxy group, a nitro group, a cyano group, an amino group, an azido group, an amidino group, a hydrazino group, a hydrazono group, a carbonyl group, a carbamyl group, a thiol group, an ester group, a carboxyl group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid or a salt thereof, a C1 to C20 alkyl group, a C2 to C20 alkenyl group, a C2 to C20 alkynyl group, a C6 to C30 aryl group, a C7 to C30 arylalkyl group, a C1 to C4 alkoxy group, a C1 to C20 heteroalkyl group, a C3 to C20 heteroarylalkyl group, a C3 to C30 cycloalkyl group, a C3 to C15 cycloalkenyl group, a C6 to C15 cycloalkynyl group, a C2 to C20 heterocycloalkyl group, and a combination thereof.

In the present specification, when a definition is not otherwise provided, “hetero” refers to one including 1 to 3 heteroatoms selected from N, O, S, and P.

In the present specification, “(meth)acrylic” refers to acrylic or methacrylic.

A separator for a lithium secondary battery according to an embodiment includes a porous substrate and a coating layer on one surface or both surfaces of the porous substrate.

The porous substrate may have a plurality of pore and may generally be a porous substrate used in an electrochemical device. Non-limiting examples of the porous substrate may be a polymer film formed of a polymer, or a copolymer or a mixture of two or more selected from polyolefin such as polyethylene, polypropylene, and the like, a polyester such as polyethylene terephthalate, polybutylene terephthalate, and the like, polyacetal, polyamide, polyimide, polycarbonate, polyetheretherketone, polyaryletherketone, polyetherimide, polyamideimide, polybenzimidazole, polyether sulfone, a polyphenylene oxide, a cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, a glass fiber, Teflon, and polytetrafluoroethylene.

The porous substrate may be, for example, a polyolefin-based substrate including polyolefin, and the polyolefin-based substrate has excellent shutdown function, thereby contributing to improvement of the safety of a battery. The polyolefin-based substrate may be, for example, selected from selected from a polyethylene single film, a polypropylene single film, a polyethylene/polypropylene double film, a polypropylene/polyethylene/polypropylene triple film, and a polyethylene/polypropylene/polyethylene triple film. In addition, the polyolefin-based resin may include a non-olefin resin in addition to an olefin resin or a copolymer of olefin and a non-olefin monomer.

The porous substrate may have a thickness of about 1 μm to 40 μm, for example 1 μm to 30 μm, 1 μm to 20 μm, 5 μm to 15 μm, or 10 μm to 15 μm.

The coating layer according to an embodiment includes a binder including a (meth)acrylic copolymer including a first structural unit derived from (meth)acrylamide, and a second structural unit including at least one of a structural unit derived from (meth)acrylic acid or (meth)acrylate, and a structural unit derived from (meth)acrylamidosulfonic acid or a salt thereof; and inorganic particles.

The first structural unit derived from the (meth)acrylamide includes an amide functional group (—NH₂) in the structural unit. The —NH₂ functional group may improve adherence characteristics of the porous substrate and the electrode, and by forming a hydrogen bond with the —OH functional group of the inorganic particles, the inorganic particles may be more firmly fixed in the coating layer, and thus the heat resistance of the separator may be strengthened.

The structural unit derived from the (meth)acrylic acid or (meth)acrylate included in the second structural unit may play a role of fixing the inorganic particles on the porous substrate, and simultaneously, provide an adhesive strength, so that a coating layer may be well adhered to the porous substrate and the electrode and accordingly, contribute to improving heat resistance and air permeability of the separator. In addition, the structural unit derived from the (meth)acrylic acid or (meth)acrylate includes a carboxyl functional group (—C(═O)O—) in the structural unit and thus may contribute to improving dispersion of coating slurry.

In addition, the structural unit derived from the (meth)acrylamidosulfonic acid or the salt thereof included in the second structural unit includes a bulky functional group, thereby reducing the mobility of the copolymer including the same and enhancing the heat resistance of the separator.

In an embodiment, the (meth)acrylic copolymer may be a two component copolymer including a first structural unit derived from (meth)acrylamide and a second structural unit derived from (meth)acrylic acid or (meth)acrylate; a two component copolymer including a first structural unit derived from (meth)acrylamide and a second structural unit derived from (meth)acrylamidosulfonic acid or a salt thereof; or a three component copolymer including a first structural unit derived from (meth)acrylamide, and a second structural unit including a structural unit derived from (meth)acrylic acid or (meth)acrylate, and a structural unit derived from (meth)acrylamidosulfonic acid or a salt thereof.

The inorganic particles may prevent a separator from being sharply shrunk due to a temperature increase. In particular, the coating layer on the separator for a lithium secondary battery according to an embodiment may include two types of inorganic particles having different average particle diameters together with the aforementioned binder including the (meth)acrylic copolymer.

When two types of inorganic particles having different average particle diameters are included, a separator having excellent binding strength for the substrate, heat resistance, and moisture resistance may be implemented compared with the case of including inorganic particles having one type of average particle diameter.

The shape of the inorganic particles according to an embodiment will be described with reference to FIG. 2.

FIG. 2 is a SEM measurement result of inorganic materials in a separator according to an embodiment.

Referring to FIG. 2, the inorganic particles may be a mixture of first inorganic particles having a relatively large average particle diameter and second inorganic particles having a relatively small average particle diameter.

The first inorganic particles may be large particle diameter inorganic particles having an average particle diameter of 400 to 600 nm.

The second inorganic particles may be small particle diameter inorganic particles having an average particle diameter of 50 to 200 nm.

The average particle diameter of the inorganic particles may be particle size (D₅₀) at a volume ratio of 50% in a cumulative size-distribution curve.

FIG. 3 is a particle size distribution curve of an inorganic material according to an embodiment.

Referring to FIG. 3, the inorganic material according to an embodiment exists in a mixed form of particles having two types of average particle diameter. The particle size distribution (PSD) of the inorganic particles according to an embodiment of the present invention may be measured using, for example, a laser diffraction method. Specifically, it may be measured by dispersing the inorganic particles in a solvent, introducing it into a commercially available laser diffraction particle size measuring device (for example, Microtrac S-3500), and irradiating an ultrasonic wave of about 28 kHz with an output of 60 W to apply a volume distribution weighted distribution method.

By including the first inorganic particles and the second inorganic particles together, inorganic particles having two types of average particle diameter may be mixed, and inorganic particles having a relatively small particle diameter may be included between inorganic particles having a relatively large particle diameter. Accordingly, a ratio of voids existing between inorganic particles may be reduced.

As a result, since the packing density of the coating layer is increased, shrinkage due to heat is reduced when left at room temperature and high temperature, so that the high temperature stability of the battery may be increased.

That is, by mixing the small particle diameter inorganic particles with the large particle diameter inorganic particles, heat resistance and moisture resistance as well as binding strength for the substrate of the separator may be improved, and thus the safety and cycle-life characteristics of the battery may be improved.

In particular, as the amount of the second inorganic particles mixed with the first inorganic particles increases, the wet heat shrinkage rate and moisture content decrease, and particularly, when the second inorganic particles are included in an amount of less than 75 wt % based on the total amount of the first inorganic particles and the second inorganic particles, the wet heat shrinkage rate and the moisture content may be minimized.

For example, the first inorganic particles and the second inorganic particles may be included in a weight ratio of 90:10 to 30:70, or a weight ratio of 90:10 to 40:60, and more specifically, the weight ratio of the first inorganic particles: the second inorganic particles may be 90:10, 75:25, 60:40, 50:50, or 40:60.

That is, the wet heat shrinkage rate and moisture content, which showed a decreasing trend with the increase of the content of the second inorganic particles, increase again when the content of the second inorganic particles exceeds 60 wt %, and when the content of the second inorganic particles exceeds 75 wt %, the binding strength for the substrate decreases and as a result, the wet heat shrinkage rate also decreases.

Therefore, when the first inorganic particles and the second inorganic particles are included within the above range, binding strength for the substrate and heat resistance may be improved, and the residual moisture content of the separator after coating is reduced, thereby minimizing side reactions caused by residual moisture when applied to a lithium secondary battery.

According to an embodiment, a moisture content of the separator including the coating layer may be less than or equal to 800 ppm, specifically less than or equal to 750 ppm, more specifically less than or equal to 730 ppm, and for example, 100 ppm to 720 ppm.

When the moisture content in the separator is within the above ranges, the cycle-life characteristics of the battery may be improved by minimizing side reactions caused by moisture.

For example, a volume ratio of the first inorganic particles and the second inorganic particles may be 12:1 to 1:7, and a specific example may be 3:1 to 1:2, for example 2:1 to 1.2:1.

When the volume ratio of the large particle diameter inorganic particles and small particle diameter inorganic particles is within the above ranges, the packing density of the coating layer increases and thus efficient thermal stability may be achieved.

For example, the packing density of the coating layer including the first inorganic particles and second inorganic particles may be 1 to 1.5 (g/μm), desirably 1.1 to 1.4 1 to 1.5 (g/μm), more desirably 1.2 to 1.4 (g/μm).

Herein, the packing density may be defined as follows.

Packing density=weight of coating layer/thickness of coating layer

The inorganic particles may be a ceramic material capable of improving heat resistance, for example, Al₂O₃, SiO₂, TiO₂, SnO₂, CeO₂, MgO, NiO, CaO, GaO, ZnO, ZrO₂, Y₂O₃, SrTiO₃, BaTiO₃, Mg(OH)₂, boehmite, or a combination thereof, but are not limited thereto. The inorganic particles may have a spherical shape, a sheet shape, a cubic shape, or an amorphous shape, and the first inorganic particles and the second inorganic particles may be the same or different.

The coating layer may include the binder including the (meth)acrylic copolymer: the first inorganic particles and the second inorganic particles in a weight ratio of 1:15 to 1:50, desirably 1:20 to 1:40, more desirably, 1:25 to 1:35. When the binder including the (meth)acrylic copolymer and inorganic particles are included in the coating layer within the above-described range, the separator may have excellent adhesive strength and heat resistance, and particularly, wet heat shrinkage properties that directly affect actual battery performance.

The first structural unit may be included in an amount of 55 mol % to 95 mol % based on 100 mol % of the (meth)acrylic copolymer, and the second structural unit may be included in an amount of 5 mol % to 45 mol % based on 100 mol % of the (meth)acrylic copolymer.

In an embodiment, the first structural unit may be included in an amount of 75 mol % to 95 mol %, for example 80 mol % to 95 mol %, based on 100 mol % of the (meth)acrylic copolymer.

Meanwhile, among the second structural units, the structural unit derived from the (meth)acrylic acid or (meth)acrylate may be included in an amount of 0 mol % to 40 mol % based on 100 mol % of the (meth)acrylic copolymer, and the structural unit derived from the (meth)acrylamidosulfonic acid or the salt thereof may be included in an amount of 0 mol % to 10 mol % based on 100 mol % of the (meth)acrylic copolymer.

For example, the structural unit derived from the (meth)acrylamide may be included in an amount of 90 mol % to 95 mol % based on 100 mol % of the (meth)acrylic copolymer, the structural unit derived from the (meth)acrylic acid or (meth)acrylate may be included in an amount of 0 mol % to 5 mol % based on 100 mol % of the (meth)acrylic copolymer, and the structural unit derived from the (meth)acrylamidosulfonic acid or the salt thereof may be included in an amount of 0 mol % to 5 mol % based on 100 mol % of the (meth)acrylic copolymer.

When the content of each structural unit is within the above range, heat resistance and adhesive strength of the separator may be further improved.

The first structural unit derived from the (meth)acrylamide may be, for example, represented by Chemical Formula 1.

In Chemical Formula 1, R¹ is hydrogen or a C1 to C6 alkyl group.

The structural unit derived from (meth)acrylic acid or (meth)acrylate may be for example represented by one of Chemical Formula 2, Chemical Formula 3, and a combination thereof.

In Chemical Formula 2 and Chemical Formula 3, R² and R³ may each independently be hydrogen or a C1 to C6 alkyl group and R⁷ may be a substituted or unsubstituted C1 to C20 alkyl group.

The structural unit derived from (meth)acrylate may be derived from (meth)acrylic acid alkyl ester, (meth)acrylic acid perfluoroalkyl ester, and (meth)acrylate having a functional group in the side chain. It may be, for example, derived from (meth)acrylate alkyl ester. In addition, the number of carbon atoms of the alkyl group or perfluoroalkyl group bound to the non-carbonyl oxygen atom of the (meth)acrylic acid alkyl ester or (meth)acrylic acid perfluoroalkyl ester may be specifically 1 to 20, more specifically 1 to 10, for example, 1 to 5.

Specific examples of the (meth)acrylic acid alkyl ester in which the carbon number of an alkyl group or a perfluoroalkyl group bound to the non-carbonyl oxygen atom is 1 to 5 may be acrylic acid alkyl ester such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, and t-butyl acrylate, and the like; 2-(perfluoroalkyl) ethyl acrylate such as 2-(perfluorobutyl) ethyl acrylate, 2-(perfluoropentyl) ethyl acrylate, and the like; methacrylic acid alkyl ester such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, and t-butyl methacrylate, and the like; and 2-(perfluoroalkyl) ethyl methacrylate such as 2-(perfluorobutyl) ethyl methacrylate, 2-(perfluoropentyl) ethyl methacrylate, 2-(perfluoroalkyl) ethyl methacrylate, and the like.

Other (meth)acrylic acid alkyl ester may be acrylic acid alkyl ester in which the carbon number of the alkyl group bound to the non-carbonyl oxygen atom is 6 to 18 such as n-hexyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, lauryl acrylate, stearyl acrylate, cyclohexyl acrylate, and isobornyl acrylate, and the like; methacrylic acid alkyl ester in which the carbon number of the alkyl group bound to the non-carbonyl oxygen atom is 6 to 18 such as n-hexyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, isodecyl methacrylate, lauryl methacrylate, tridecyl methacrylate, stearyl methacrylate, and cyclohexyl methacrylate; 2-(perfluoroalkyl) ethyl acrylate in which the carbon number of the perfluoroalkyl group bound to the non-carbonyl oxygen atom is 6 to 18 such as 2-(perfluorohexyl)ethyl acrylate, 2-(perfluorooctyl) ethyl acrylate, 2-(perfluorononyl) ethyl acrylate, 2-(perfluorodecyl) ethyl acrylate, 2-(perfluorododecyl) ethyl acrylate, 2-(perfluorotetradecyl) ethyl acrylate, 2-(perfluorohexadecyl) ethyl acrylate, and the like; 2-(perfluoroalkyl) ethyl methacrylate in which the carbon number of the perfluoroalkyl group bound to the non-carbonyl oxygen atom is 6 to 18 such as 2-(perfluorohexyl) ethyl methacrylate, 2-(perfluorooctyl) ethyl methacrylate, 2-(perfluorononyl) ethyl methacrylate, 2-(perfluorodecyl) ethyl methacrylate, 2-(perfluorododecyl) ethyl methacrylate, 2-(perfluorotetradecyl) ethyl methacrylate, 2-(perfluorohexadecyl) ethyl methacrylate, and the like.

For example, the structural unit derived from (meth)acrylic acid or (meth)acrylate includes a structural unit represented by Chemical Formula 2 and a structural unit represented by Chemical Formula 3 respectively or both of them together, and when the structural units are included together, the structural units represented by Chemical Formulas 2 and 3 may be included in a mole ratio of 10:1 to 1:1, specifically, 6:1 to 1:1, and more specifically, 3:1 to 1:1.

The structural unit derived from (meth)acrylamidosulfonic acid or a salt thereof may be a structural unit derived from (meth)acrylamido sulfonic acid or (meth)acrylamido sulfonate, wherein the (meth)acrylamido sulfonate may be a conjugate base of the (meth)acrylamido sulfonic acid, a (meth)acrylamido sulfonate salt, or a derivative thereof. The structural unit derived from the (meth)acrylamidosulfonic acid or (meth)acrylamidosulfonate may be, for example, represented by one of Chemical Formula 4, Chemical Formula 5, Chemical Formula 6, and a combination thereof.

In Chemical Formula 4 to Chemical Formula 6, R⁴, R⁵ and R⁶ are each independently hydrogen or a C1 to C6 alkyl group, L¹, L², and L³ are each independently a substituted or unsubstituted C1 to C10 alkylene group, a substituted or unsubstituted C3 to C20 cycloalkylene group, a substituted or unsubstituted C6 to C20 arylene group, or a substituted or unsubstituted C3 to C20 heterocyclic group, a, b, and c are each independently an integer of 0 to 2, and M is an alkali metal, wherein the alkali metal may be, for example, lithium, sodium, potassium, rubidium, or cesium.

For example, in Chemical Formulas 4 to 6, L¹, L², and L³ may each independently be a substituted or unsubstituted C1 to C10 alkylene group, and a, b and c are each 1.

The structural unit derived from the (meth)acrylamidosulfonic acid and salt thereof may include each of the structural unit represented by Chemical Formula 4, the structural unit represented by Chemical Formula 5, and the structural unit represented by Chemical Formula 6, or may include two or more types thereof together. As one example, the structural unit represented by Chemical Formula 5 may be included, and as another example, the structural unit represented by Chemical Formula 5 and the structural unit represented by Chemical Formula 6 may be included together.

When the structural unit represented by Chemical Formula 5 and the structural unit represented by Chemical Formula 6 are included together, the structural unit represented by Chemical Formula 5 and the structural unit represented by Chemical Formula 6 may be included in a mole ratio of 10:1 to 1:2, desirably 5:1 to 1:1, and more desirably 3:1 to 1:1.

The sulfonate group in the structural unit derived from (meth)acrylamidosulfonic acid or the salt thereof may be, for example, a functional group derived from vinyl sulfonic acid, allyl sulfonic acid, styrene sulfonic acid, anethol sulfonic acid, acrylamidoalkane sulfonic acid, sulfoalkyl (meth)acrylate, or a salt thereof.

Herein, the alkane may be a C1 to C20 alkane, a C1 to C10 alkane, or a C1 to C6 alkane, and the alkyl may be a C1 to C20 alkyl, a C1 to C10 alkyl, or a C1 to C6 alkyl. The salt refers to a salt composed of the aforementioned sulfonic acid and appropriate ions. The ions may be, for example, alkali metal ions, and in this case, the salt may be an alkali metal sulfonate salt.

The acrylamidoalkane sulfonic acid may be, for example, 2-acrylamido-2-methylpropane sulfonic acid and the sulfoalkyl (meth)acrylate may be, for example, 2-sulfoethyl (meth)acrylate, 3-sulfopropyl (meth)acrylate, and the like.

The (meth)acrylic copolymer may be represented by Chemical Formula 7.

In Chemical Formula 7, R⁸ to R¹⁰ are each independently hydrogen or a methyl group, R¹¹ is hydrogen or a C1 to C6 alkyl group, L² is a substituted or unsubstituted C1 to C10 alkylene group, a substituted or unsubstituted C3 to C20 cycloalkylene group, a substituted or unsubstituted C6 to C20 arylene group, or a substituted or unsubstituted C3 to C20 heterocyclic group, b is one of integers of 0 to 2, M is an alkali metal such as lithium, sodium, potassium, rubidium, cesium, and the like, and l, m, and n indicate mole ratios of each unit.

For example, in Chemical Formula 7, l+m+n may be 1. In addition, for example, it may be 0.05≤(l+n)≤0.45, 0.55≤m≤0.95, specifically 0≤l≤0.4, and 0≤n≤0.1, for example 0.9≤m≤0.95, 0≤l≤0.05, and 0≤n≤0.05.

For example, in Chemical Formula 7, L² may be a substituted or unsubstituted C1 to C10 alkylene group, and b may be 1.

In the (meth)acrylicacrylic copolymer, the structural unit substituted with the alkali metal (M+) may be 50 mol % to 100 mol %, for example 60 mol % to 90 mol % or 70 mol % to 90 mol % based on a total amount, 100 mol %, of the (meth)acrylamido sulfonic acid structural unit. When the ranges are satisfied, the (meth)acrylic copolymer and the separator including the same may exhibit improved adhesive strength, heat resistance, and oxidation resistance.

The (meth)acrylic copolymer may further include other units in addition to the units. For example the (meth)acrylic copolymer may further include a unit derived from alkyl(meth)acrylate, a unit derived from a diene-based monomer, a unit derived from a styrene-based monomer, an ester group-containing unit, a carbonate group-containing unit, or a combination thereof.

The (meth)acrylic copolymer may have various forms, that is, an alternate polymer where the units are alternately distributed, a random polymer the units are randomly distributed, or a graft polymer where a part of structural unit is grafted.

A weight average molecular weight of the (meth)acrylic copolymer may be 350,000 to 970,000, for example 450,000 to 970,000, or 450,000 to 700,000. When the weight average molecular weight of the (meth)acrylic copolymer satisfies the ranges, the (meth)acrylic copolymer and the separator including the same may exhibit excellent adhesive strength, heat resistance, and air permeability. The weight average molecular weight may be polystyrene-reduced average molecular weight measured by gel permeation chromatography.

The (meth)acrylic copolymer may be prepared by various methods such as emulsion polymerization, suspension polymerization, massive polymerization, solution polymerization, or bulk polymerization.

The coating layer may have a thickness of about 1 μm to 5 μm, for example 1.5 μm to 3 μm.

A ratio of a thickness of the coating layer to a thickness of the porous substrate may be 0.05 to 0.5, for example 0.05 to 0.4, or 0.05 to 0.3, or 0.1 to 0.2. In this case, the separator including the porous substrate and the coating layer may exhibit excellent air permeability, heat resistance, and adhesive strength.

The separator for a lithium secondary battery according to an embodiment has excellent heat resistance. Specifically, the separator may have a shrinkage rate of less than 30%, or less than or equal to 25%. For example, after the separator is left at 150° C. for 60 minutes, the dry shrinkage rates and wet shrinkage rates in the longitudinal direction and the transverse direction of the separator measured may be less than or equal to 5% and less than or equal to 20%, respectively.

When a stability issue such as ignition occurs in the actual battery, the heat shrinkage rate in the electrolyte at high temperature affects the battery characteristics, and thus since the wet shrinkage rate becomes a more important factor, the wet shrinkage rate should be improved in addition to the dry shrinkage rate.

The separator for a lithium secondary battery according to an embodiment may exhibit excellent air permeability, and specifically less than 160 sec/100 cc·1 μm, for example less than or equal to 150 sec/100 cc·1 μm, or less than or equal to 140 sec/100 cc·1 μm per unit thickness. Herein, the air permeability refers to the time (seconds) it takes for 100 cc of air to pass through the unit thickness of the separator. The air permeability per unit thickness may be obtained by measuring the air permeability for the total thickness of the separator and dividing it by the thickness.

The separator for a lithium secondary battery according to an embodiment may be manufactured by known various methods. For example, the separator for a lithium secondary battery may be formed by coating a composition for forming a coating layer on one surface or both surfaces of the porous substrate and drying the same.

The composition for forming the coating layer may include a binder including the aforementioned (meth)acrylic copolymer, first inorganic particles, second inorganic particles, and a solvent. The solvent is not particularly limited if the solvent may dissolve or disperse the binder including a (meth)acrylic copolymer and the inorganic particles. In an embodiment, the solvent may be an aqueous solvent including water, an alcohol, or a combination thereof, which is environmentally-friendly.

Meanwhile, the binder may further include a crosslinked binder having a crosslinked structure in addition to the (meth)acrylic copolymer. The crosslinked binder may be obtained from a monomer, an oligomer, and/or a polymer having a curable functional group capable of reacting with heat and/or light, for example, a multi-functional monomer, a multi-functional oligomer, and/or a multi-functional polymer having at least two curable functional groups. The curable functional group may include a vinyl group, a (meth)acrylate group, an epoxy group, an oxetane group, an ether group, a cyanate group, an isocyanate group, a hydroxy group, a carboxyl group, a thiol group, an amino group, an alkoxy group, or a combination thereof but is not limited thereto.

The crosslinked binder may be for example, obtained by curing a monomer, an oligomer and/or a polymer including at least two (meth)acrylate groups, and may be for example obtained by curing ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, butanediol di(meth)acrylate, hexamethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerine tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, diglycerine hexa(meth)acrylate, or a combination thereof.

For example, the crosslinked binder may be obtained from a monomer, an oligomer and/or a polymer including at least two epoxy groups, for example bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, hexahydrophthalic acid glycidyl ester, or a combination thereof.

For example, the crosslinked binder may be obtained from a monomer, an oligomer and/or a polymer including at least two isocyanate groups, for example diphenylmethane diisocyanate, 1,6-hexamethylene diisocyanate, 2,2,4(2,2,4)-trimethyl hexamethylene diisocyanate, phenylene diisocyanate, 4,4′-dicyclohexylmethane diisocyanate, 3,3′-dimethyldiphenyl-4,4′-diisocyanate, xylene diisocyanate, naphthalene diisocyanate, 1,4-cyclohexyl diisocyanate, or a combination thereof.

In addition, the binder may further include a non-crosslinked binder in addition to the (meth)acrylic copolymer. The non-crosslinked binder may be for example a vinylidenefluoride-based polymer, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, a polyethylene-vinylacetate copolymer, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxylmethyl cellulose, an acrylonitrile-styrene-butadiene copolymer, or a combination thereof, but is not limited thereto.

The vinylidene fluoride-based polymer may be specifically a homopolymer including only vinylidene fluoride monomer-derived unit or a copolymer of a vinylidene fluoride-derived unit and other monomer-derived unit. The copolymer may specifically include a vinylidene fluoride-derived unit and at least one of units derived from chlorotrifluoroethylene, trifluoroethylene, hexafluoropropylene, ethylene tetrafluoride and ethylene monomers, but is not limited thereto. For example, the copolymer may be a polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) copolymer including a vinylidene fluoride monomer-derived unit and a hexafluoropropylene monomer-derived unit.

For example, the non-crosslinked binder may be a polyvinylidene fluoride (PVdF) homopolymer, a polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) copolymer, or a combination thereof. In this case, an adhesive strength between the porous substrate and the coating layer is increased, stability of the separator and impregnation properties of an electrolyte solution are improved, and thus high-rate charge and discharge characteristics of a battery is improved.

The coating may be, for example spin coating, dip coating, bar coating, die coating, slit coating, roll coating, inkjet printing, and the like, but is not limited thereto.

The drying may be for example performed through natural drying, drying with warm air, hot air, or low humid air, vacuum-drying, or radiation of a far-infrared ray, an electron beam, and the like, but the present disclosure is not limited thereto. The drying may be for example performed at a temperature of 25° C. to 120° C.

The separator for a lithium secondary battery may be manufactured by lamination, coextrusion, and the like besides the above method.

Hereinafter, a lithium secondary battery including the separator for a lithium secondary battery will be described.

A lithium secondary battery may be classified into a lithium ion battery, a lithium ion polymer battery, and a lithium polymer battery depending on kinds of a separator and an electrolyte and also may be classified to be cylindrical, prismatic, coin-type, pouch-type, and the like depending on shape. In addition, it may be bulk type and thin film type depending on sizes. Structures and manufacturing methods for lithium ion batteries pertaining to this disclosure are well known in the art.

Herein, as an example of a lithium secondary battery, a cylindrical lithium secondary battery is exemplarily described. FIG. 1 is an exploded perspective view of a lithium secondary battery according to an embodiment. Referring to FIG. 1, a lithium secondary battery 100 according to an embodiment includes a battery cell including a negative electrode 112, a positive electrode 114 facing the negative electrode 112, a separator 113 between the negative electrode 112 and the positive electrode 114, and an electrolyte (not shown) impregnating the negative electrode 112, the positive electrode 114 and the separator 113, and a battery container 120, a battery case containing the battery cell, and a sealing member 140 that seals the container 120.

The positive electrode 114 may include a positive current collector and a positive active material layer formed on the positive current collector. The positive active material layer includes a positive active material, a binder, and optionally a conductive material.

The positive current collector may use aluminum (Al), nickel (Ni), and the like, but is not limited thereto.

The positive active material may use a compound capable of intercalating and deintercallating lithium. Specifically at least one of a composite oxide or a composite phosphate of a metal selected from cobalt, manganese, nickel, aluminum, iron, or a combination thereof and lithium may be used. More specifically, the positive active material may use lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, or a combination thereof.

The binder improves binding properties of positive active material particles with one another and with a current collector. Specific examples may be polyvinyl alcohol, carboxylmethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, an ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, a styrene-butadiene rubber, an acrylated styrene-butadiene rubber, an epoxy resin, nylon, and the like, but are not limited thereto. These may be used alone or as a mixture of two or more.

The conductive material improves conductivity of an electrode. Examples thereof may be natural graphite, artificial graphite, carbon black, a carbon fiber, a metal powder, a metal fiber, and the like, but are not limited thereto. These may be used alone or as a mixture of two or more. The metal powder and the metal fiber may use a metal of copper, nickel, aluminum, silver, and the like.

The negative electrode 112 includes a negative current collector and a negative active material layer formed on the negative current collector.

The negative current collector may use copper, gold, nickel, a copper alloy, but is not limited thereto.

The negative active material layer may include a negative active material, a binder and optionally a conductive material. The negative active material may be a material that reversibly intercalates/deintercalates lithium ions, a lithium metal, a lithium metal alloy, a material capable of doping and dedoping lithium, a transition metal oxide, or a combination thereof.

The material that reversibly intercalates/deintercalates lithium ions may be a carbon material which is any generally-used carbon-based negative active material, and examples thereof may be crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon may be graphite such as amorphous, sheet-shaped, flake-shaped, spherically, shaped or fiber-shaped natural graphite or artificial graphite. Examples of the amorphous carbon may be soft carbon or hard carbon, a mesophase pitch carbonized product, fired coke, and the like. The lithium metal alloy may be an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn. The material capable of doping and dedoping lithium may be Si, SiO_(x) (0<x<2), a Si—C composite, a Si—Y alloy, Sn, SnO₂, a Sn—C composite, a Sn—Y alloy, and the like, and at least one of these may be mixed with SiO₂. The element Y may be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, TI, Ge, P, As, Sb, Bi, S, Se, Te, Po, and a combination thereof. The transition metal oxide may be vanadium oxide, lithium vanadium oxide, and the like.

The binder and the conductive material used in the negative electrode 112 may be the same as the binder and conductive material of the positive electrode 114.

The positive electrode 114 and the negative electrode 112 may be manufactured by mixing each active material composition including each active material and a binder, and optionally a conductive material in a solvent, and coating the active material composition on each current collector. Herein, the solvent may be N-methylpyrrolidone, and the like, but is not limited thereto. The electrode manufacturing method is well known, and thus is not described in detail in the present specification.

The electrolyte includes an organic solvent a lithium salt.

The organic solvent serves as a medium for transmitting ions taking part in the electrochemical reaction of a battery. Examples thereof may be selected from a carbonate-based solvent, an ester-based solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent, and an aprotic solvent. The carbonate-based solvent may be dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methylethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, and the like, and the ester-based solvent may be methyl acetate, ethyl acetate, n-propyl acetate, 1,1-dimethylethyl acetate, methylpropionate, ethylpropionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, and the like. The ether-based solvent may be dibutyl ether, tetraglyme, dig lym e, dim ethoxyethane, 2-m ethyltetrahydrofuran, tetrahydrofuran, and the like, and the ketone-based solvent may be cyclohexanone, and the like. The alcohol-based solvent may be ethanol, isopropyl alcohol, and the like, and the aprotic solvent may be nitriles such as R—CN (R is a C2 to C20 linear, branched, or cyclic hydrocarbon group, a double bond, an aromatic ring, or an ether bond), and the like, amides such as dimethyl formamide, dioxolanes such as 1,3-dioxolane, sulfolanes, and the like.

The organic solvent may be used alone or in a mixture of two or more, and when the organic solvent is used in a mixture of two or more, the mixture ratio may be controlled in accordance with a desirable cell performance.

The lithium salt is dissolved in an organic solvent, supplies lithium ions in a battery, basically operates the lithium secondary battery, and improves lithium ion transportation between positive and negative electrodes therein. Examples of the lithium salt may include two or more selected from LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiN(SO₃C₂F₅)₂, LiN(CF₃SO₂)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂, LiAlCl₄, LiN(C_(x)F_(2x+1)SO₂)(C_(y)F₂₊₁SO₂), wherein, x and y are natural numbers, LiCl, Lil, LiB(C₂O₄)₂, or a combination thereof, but are not limited thereto.

The lithium salt may be used in a concentration ranging from 0.1 M to 2.0 M. When the lithium salt is included within the above concentration range, an electrolyte may have excellent performance and lithium ion mobility due to optimal electrolyte conductivity and viscosity.

Hereinafter, the above aspects of the present invention are illustrated in more detail with reference to examples. However, these examples are exemplary, and the present disclosure is not limited thereto.

Synthesis Example: Preparation of (Meth)acrylic Copolymer Synthesis Example 1

Distilled water (6361 g), acrylic acid (72.06 g, 1.0 mol), acrylamide (604.1 g, 8.5 mol), potassium persulfate (2.7 g, 0.01 mol), 2-acrylamido-2-methylpropanesulfonic acid (103.6 g, 0.5 mol), and a 5 N lithium hydroxide aqueous solution (1.05 equivalent based on a total amount of 2-acrylamido-2-methylpropanesulfonic acid) were added to a 10 L four-necked flask equipped with a stirrer, a thermometer, and a cooling tube, and an operation of reducing the internal pressure to 10 mmHg with a diaphragm pump and then, returning it to a normal pressure with nitrogen was repeated three times.

The reaction solution was reacted for 12 hours while adjusting the temperature of the reaction solution to be stabilized between 65° C. and 70° C. After cooling to room temperature, the pH of the reaction solution was adjusted to 7 to 8 using a 25% aqueous ammonia solution.

By this method, poly(acrylic acid-co-acrylamide-co-2-acrylamido-2-methylpropanesulfonic acid) sodium salt was prepared. A mole ratio of acrylic acid, acrylamide, and 2-acrylamido-2-methylpropanesulfonic acid was 10:85:5. About 10 mL of the reaction solution (reaction product) was taken to measure the non-volatile component, and the result was 9.5% (theoretical value: 10%).

Examples: Manufacture of Separator for Lithium Secondary Battery Example 1

The acrylic copolymer (10 wt % in distilled water) according to Synthesis Example 1: inorganic particles obtained by mixing the first inorganic particles (boehmite, manufactured by Anhui Estone, average particle diameter 500 nm) and the second inorganic particles (alumina, manufactured by Evonik, average particle diameter 100 nm) at a weight ratio of 90:10 were added to a water solvent at a weight ratio of 1:25 to prepare organic-inorganic dispersion by milling and dispersing at 25° C. for 30 minutes using a bead mill. Subsequently, water was added thereto so that a total solid content might be 25 wt % to prepare a composition for a coating layer. The composition was coated to be 3-μm thick on a 8 μm-thick polyethylene porous substrate (air permeability: 120 sec/100 cc, puncture strength: 480 kgf, SK global chemical Co., Ltd.) by a bar-coating method and then, dried at 70° C. for 10 minutes to manufacture a separator for a lithium secondary battery.

Example 2

A separator for a lithium secondary battery was manufactured in the same manner as in Example 1, except that inorganic particles obtained by mixing the first inorganic particles and the second inorganic particles at a weight ratio of 75:25 were added.

Example 3

A separator for a lithium secondary battery was manufactured in the same manner as in Example 1, except that inorganic particles obtained by mixing the first inorganic particles and the second inorganic particles at a weight ratio of 60:40 were added.

Example 4

A separator for a lithium secondary battery was manufactured in the same manner as in Example 1, except that inorganic particles obtained by mixing the first inorganic particles and the second inorganic particles at a weight ratio of 50:50 were added.

Example 5

A separator for a lithium secondary battery was manufactured in the same manner as in Example 1, except that inorganic particles obtained by mixing the first inorganic particles and the second inorganic particles at a weight ratio of 40:60 were added.

Example 6

A separator for a lithium secondary battery was prepared in the same manner as in Example 4, except that the composition was coated to be 1.5-μm thick on both surfaces of a 8 μm-thick polyethylene porous substrate (SK global chemical Co., Ltd., air permeability: 120 sec/100 cc, puncture strength: 480 kgf) by a die coating method.

Comparative Example 1

A separator for a secondary battery was prepared in the same manner as in Example 1, except that the second inorganic particles were not added (the first inorganic particles: the second inorganic particles=100:0 by weight).

Reference Example 1

A separator for a lithium secondary battery was manufactured in the same manner as in Example 1, except that inorganic particles obtained by mixing the first inorganic particles and the second inorganic particles at a weight ratio of 25:75 were added.

Reference Example 2

A separator for a lithium secondary battery was manufactured in the same manner as in Example 1, except that inorganic particles obtained by mixing the first inorganic particles and the second inorganic particles at a weight ratio of 10:90 were added.

Comparative Example 2

A separator for a secondary battery was prepared in the same manner as in Example 1, except that the first inorganic particles were not added (the first inorganic particles: the second inorganic particles=0:100 by weight).

Comparative Example 3

A separator for a secondary battery was prepared in the same manner as in Example 6, except that the second inorganic particles were not added (the first inorganic particles: the second inorganic particles=100:0 by weight).

Comparative Examples 4 to 8

A separator for a secondary battery was manufactured in the same manner as in Example 1, except that an acrylic copolymers according to the compositions shown in Table 2 were used instead of the acrylic copolymer prepared in Synthesis Example 1.

EVALUATION EXAMPLES Evaluation Example 1: Adhesive Strength to Substrate

The separators according to Examples 1 to 6, Reference Examples 1 and 2, and Comparative Examples 1 to 3 were respectively cut into a size having a width of 12 mm and a length of 50 mm to prepare each sample. After adhering a tape to the coating layer surface of each sample, separating the tape-adhered surface from the substrate about 10 mm to 20 mm apart, and then, clipping the substrate having no tape with an upper grip, the coating layer surface adhered with the tape with a lower grip, and fixing a distance between the grips into 20 mm, the coating layer was pulled toward a direction of 180° and peeled off from each separator. Herein, strength for peeling off up to 40 mm at a peeling speed of 10 mm/min was three times measured and averaged. The peeling strength results are shown in Table 1.

Evaluation Example 2: Heat Shrinkage Rate

(Dry Heat Shrinkage Rate)

The separators for a lithium secondary battery according to Examples 1 to 6, Reference Example 1 and 2 and Comparative Example 1 to 8 were respectively cut into a size of 8 cm×8 cm to prepare samples. The samples after drawing a 5 cm×5 cm-size quadrangle on the surface were inserted between paper or alumina powder, and then after being left at 150° C. in an oven for 1 hour, the samples were taken out of the oven, and sides of the quadrangles drawn on the samples were measured to calculate each shrinkage rate in machine direction (MD) and in a traverse direction (TD). The results are shown in Tables 1 and 2.

(Wet Heat Shrinkage Rate)

The separators for a lithium secondary battery according to Examples 1 to 6, Reference Examples 1 and 2 and Comparative Examples 1 to 8 were respectively cut into a size of 5 cm×5 cm to prepare samples. The samples were inserted into polyethylene terephthalate release paper and were put in an aluminum pouch, 2 g of electrolyte was injected and sealed, and then after being left in an oven at 150° C. for 1 hour, the samples were taken out and the horizontal and vertical dimensions were measured to calculate each shrinkage rate in a machine direction (MD) and a traverse direction (TD). The results are shown in Tables 1 and 2 and FIGS. 4 and 5.

Evaluation Example 3: Evaluation of Moisture Content of Separator

The moisture contents of the separators for a lithium secondary battery according to Examples 1 to 6, Reference Examples 1 and 2, and Comparative Examples 1 to 3 were evaluated using a Karl Fisher method, and the results are shown in Table 1.

TABLE 1 Binder (acrylic copolymer):inorganic material (weight ratio) 1:25 Dry heat Wet heat First shrinkage shrinkage inorganic rate rate particle:second 150° C., 150° C., inorganic Coating Adhesive 1 hr 1 hr Moisture Coating particle thickness strength MD TD MD TD content surface (weight ratio) (μm) (gf) (%) (%) (%) (%) (ppm) Ex. 1 90:10 3 11.1 1.4 1.6 19 19 381 Single Ex. 2 75:25 3 12.0 1.3 1.4 17 18 415 surface Ex. 3 60:40 3 12.5 1.2 1.5 14 13 459 Ex. 4 50:50 3 13.6 1.0 1.5 10 9 488 Ex. 5 40:60 3 12.8 1.1 1.7 18 19 711 Comp. Ex. 1 100:0  3 10.1 1.1 1.0 21 21 317 Reference Ex. 1 25:75 3 12.3 1.3 2.2 25 23 892 Reference Ex. 2 10:90 3 11.4 1.5 1.8 27 24 1101 Comp. Ex. 2  0:100 3 10.2 1.8 1.6 35 25 1591 Both Ex. 6 50:50 3 15.6 0.5 0.6 5 6 1.40 surfaces Comp. Ex. 3 100:0  3 14.7 0.8 0.9 17 19 1.15

FIG. 5 is a graph showing wet heat shrinkage in a transverse direction (TD) according to the content of the second inorganic particles.

Referring to Table 1 and FIG. 5, the separators including a bimodal system inorganic material according to an embodiment had excellent adhesive strength to the substrate and simultaneously, exhibited a greatly improved wet heat shrinkage rate having an influence on actual cell characteristics, when the inorganic particles were mixed in a specific ratio.

In other words, referring to Table 1 and FIG. 5, the separators including a bimodal system inorganic material according to Examples 1 to 6 and Reference Examples 1 and 2 had excellent adhesive strength to the substrate, and, particularly, the separators including 60 wt % or more of the second inorganic particles according to Examples 1 to 6 exhibited a greatly improved wet heat shrinkage rate, which is a heat shrinkage rate in an electrolyte solution, and also a greatly improved moisture content, and accordingly, heat resistance thereof in actual cells may also be expected to be greatly improved.

TABLE 2 Dry heat Wet heat First shrinkage shrinkage Acrylic inorganic rate rate copolymer particles:second Coating 150° C., 1 hr 150° C., 1 hr (acrylate inorganic thickness MD TD MD TD content %) particles (μm) (%) (%) (%) (%) Ex. 2 Synthesis Example 1 75:25 3 1.3 1.4 17 18 Comp. AA(20)/CMC 75:25 3 5.1 5.2 65 68 Ex. 4 Comp. AA(30)/VP(70) 75:25 3 1.5 1.6 60 51 Ex. 5 Comp. AA(40)/CMC(20)/VP(40) 75:25 3 1.5 2.1 61 61 Ex. 6 Comp. AA(50)/VA(50) 75:25 3 1.2 1.6 61 65 Ex. 7 Comp. AA(100) 75:25 3 1.5 1.6 65 70 Ex. 8 (AA: structural unit derived from acrylic acid, CMC: structural unit derived from carboxymethyl cellulose, VP: structural unit derived from N-vinyl-pyrrolidone, and VA: structural unit derived from vinyl alcohol)

FIG. 4 is a photograph showing a change result after wet shrinkage of the coated separator according to Comparative Examples 4 to 8.

Referring to Table 2 and FIG. 4, the examples exhibited equivalent dry heat shrinkage rates to those of comparative examples but greatly improved wet heat shrinkage rates, which are heat shrinkage rates in an electrolyte solution, and accordingly, there may be a large difference in heat resistance when applied to actual cells.

While this invention has been described in connection with what is presently considered to be practical embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A separator for a lithium secondary battery, comprising a porous substrate, and a coating layer on at least one surface of the porous substrate, wherein the coating layer comprises a binder including a (meth)acrylic copolymer including a first structural unit derived from (meth)acrylamide, and a second structural unit including at least one of a structural unit derived from (meth)acrylic acid or (meth)acrylate, and a structural unit derived from (meth)acrylamidosulfonic acid or a salt thereof; first inorganic particles; and second inorganic particles, wherein the first inorganic particles have an average particle diameter of 400 to 600 nm, and the average particle diameter of the second inorganic particles is smaller than the average particle diameter of the first inorganic particles.
 2. The separator of claim 1, wherein the second inorganic particles are included in an amount of less than 75 wt % based on the total amount of the first inorganic particles and the second inorganic particles.
 3. The separator of claim 1, wherein the first inorganic particles and the second inorganic particles are included in a weight ratio of 90:10 to 40:60.
 4. The separator of claim 1, wherein the second inorganic particles have an average particle diameter of 50 to 200 nm.
 5. The separator of claim 1, wherein a volume ratio of the first inorganic particles and the second inorganic particles is 12:1 to 1:7.
 6. The separator of claim 1, wherein the first inorganic particles and the second inorganic particles each independently comprise Al₂O₃, SiO₂, TiO₂, SnO₂, CeO₂, MgO, NiO, CaO, GaO, ZnO, ZrO₂, Y₂O₃, SrTiO₃, BaTiO₃, Mg(OH)₂, boehmite, or a combination thereof.
 7. The separator of claim 1, wherein a weight ratio of the binder: first inorganic particles and second inorganic particles is 1:15 to 1:50.
 8. The separator of claim 1, wherein the first structural unit is included in an amount of 55 mol % to 95 mol % based on 100 mol % of the (meth)acrylic copolymer, and the second structural unit is included in an amount of 5 mol % to 45 mol % based on 100 mol % of the (meth)acrylic copolymer.
 9. The separator of claim 1, wherein the structural unit derived from the (meth)acrylic acid or (meth)acrylate is included in an amount of 0 mol % to 40 mol % based on 100 mol % of the (meth)acrylic copolymer, and the structural unit derived from the (meth)acrylamidosulfonic acid or the salt thereof is included in an amount of 0 mol % to 10 mol % based on 100 mol % of the (meth)acrylic copolymer.
 10. The separator of claim 1, wherein the structural unit derived from the (meth)acrylamide is included in an amount of 90 mol % to 95 mol % based on 100 mol % of the (meth)acrylic copolymer, the structural unit derived from the (meth)acrylic acid or (meth)acrylate is included in an amount of 0 mol % to 5 mol % based on 100 mol % of the (meth)acrylic copolymer, and the structural unit derived from the (meth)acrylamidosulfonic acid or the salt thereof is included in an amount of 0 mol % to 5 mol % based on 100 mol % of the (meth)acrylic copolymer.
 11. The separator of claim 1, wherein the first structural unit derived from (meth)acrylamide is represented by Chemical Formula 1, the structural unit derived from the (meth)acrylic acid or (meth)acrylate is represented by one of Chemical Formula 2, Chemical Formula 3, and a combination thereof, and the structural unit derived from the (meth)acrylamidosulfonic acid or the salt thereof is represented by one of Chemical Formula 4, Chemical Formula 5, Chemical Formula 6, and a combination thereof:

wherein, in Chemical Formula 1 to Chemical Formula 6, R¹ to R⁶ are each independently hydrogen or a C1 to C6 alkyl group, R⁷ is a substituted or unsubstituted C1 to C20 alkyl group, L¹ to L³ are each independently a substituted or unsubstituted C1 to C10 alkylene group, a substituted or unsubstituted C3 to C20 cycloalkylene group, a substituted or unsubstituted C6 to C20 arylene group, or a substituted or unsubstituted C3 to C20 heterocyclic group, a, b, and c are each independently one of integers of 0 to 2, and M is an alkali metal.
 12. The separator of claim 1, wherein a weight average molecular weight of the (meth)acrylic copolymer is 350,000 to 970,000.
 13. The separator of claim 1, wherein the separator has a moisture content of less than or equal to 800 ppm.
 14. The separator of claim 1, wherein the coating layer has a thickness of 1 μm to 5 μm.
 15. A lithium secondary battery, comprising a positive electrode, a negative electrode, and the separator of claim 1 between the positive electrode and the negative electrode. 